NEW OPERATING MODE OF A QUADRUPOLE ION-TRAP IN MASS-SPECTROMETRY .3. MASS RESOLUTION

A theoretical expression for the mass resolution relating to a new ope rating mode of a quadrupole ion trap in mass spectrometry (described i n two previous papers Y. Zerega, J. Andre, G. Brincourt and R. Catella , Int. J. Mass Spectrom Ion Processes, 132 (1994) 57; 132 (1994) 67) i s based on an analysis of the time-of-flight of ejected ions after con finement. For a given operating point, the resolution is proportional to two factors: the number, N, of confinement temporal samplings and t he ratio j of the sampling period to the period of the confinement fie ld. The value of N partially defines the duration of the experiment wh ich is roughly proportional to N2. Too high a value of N can lead to a n overly long duration of the experiment. For a given sampling period, T(e), the higher secular frequency, which corresponds to the lower en d of the mass range to be analysed, j is defined by the Shannon criter ion. This criterion also assigns the value zero to the lower secular f requency corresponding to an infinitely high value of the mass. All th is limits the value of j. But we show that for a finite value of mass at the higher end of the mass range, which is the event experimentally , sub-sampling is possible leading to higher values of j than previous ly. In any event, resolution is limited by neutral/ion collisions and by imperfections of the quadrupolar confinement field both of which pe rturb the motion of the ions and hence enlarge the frequency peaks. Th e present experimental results lead to a value for the mass resolution of 400 at full width at half maximum (FWHM) for m/Z = 132 Da. With ex perimental improvements (a confinement frequency of 1.0 MHz, a pressur e of less than 10(-7) Torr and a defect-free trap), we theoretically e xpect a mass resolution of about 10(5) at FWHM.